Re: [PATCH v2 08/11] hugetlb: batch freeing of vmemmap pages

[Muchun Song]

> On Sep 7, 2023, at 05:38, Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
> 
> On 09/06/23 15:38, Muchun Song wrote:
>> 
>> 
>> On 2023/9/6 05:44, Mike Kravetz wrote:
>>> Now that batching of hugetlb vmemmap optimization processing is possible,
>>> batch the freeing of vmemmap pages.  When freeing vmemmap pages for a
>>> hugetlb page, we add them to a list that is freed after the entire batch
>>> has been processed.
>>> 
>>> This enhances the ability to return contiguous ranges of memory to the
>>> low level allocators.
>>> 
>>> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
>>> ---
>>>  mm/hugetlb_vmemmap.c | 60 ++++++++++++++++++++++++++++----------------
>>>  1 file changed, 38 insertions(+), 22 deletions(-)
>>> 
>>> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
>>> index 79de984919ef..a715712df831 100644
>>> --- a/mm/hugetlb_vmemmap.c
>>> +++ b/mm/hugetlb_vmemmap.c
>>> @@ -306,18 +306,21 @@ static void vmemmap_restore_pte(pte_t *pte, unsigned long addr,
>>>   * @end: end address of the vmemmap virtual address range that we want to
>>>   * remap.
>>>   * @reuse: reuse address.
>>> + * @vmemmap_pages: list to deposit vmemmap pages to be freed.  It is callers
>>> + * responsibility to free pages.
>>>   *
>>>   * Return: %0 on success, negative error code otherwise.
>>>   */
>>>  static int vmemmap_remap_free(unsigned long start, unsigned long end,
>>> -       unsigned long reuse)
>>> +       unsigned long reuse,
>>> +       struct list_head *vmemmap_pages)
>>>  {
>>>   int ret;
>>> - LIST_HEAD(vmemmap_pages);
>>> + LIST_HEAD(freed_pages);
>> 
>> IIUC, we could reuse the parameter of @vmemmap_pages directly instead of
>> a temporary variable, could it be dropped?
>> 
> 
> I was concerned about the error case where we call vmemmap_remap_range a
> second time.  In the first call to vmemmap_remap_range with vmemmap_remap_pte,
> vmemmap pages to be freed are added to the end of the list (list_add_tail).
> In the call to vmemmap_remap_range after error with vmemmap_restore_pte,
> pages are taken off the head of the list (list_first_entry).  So, it seems
> that it would be possible to use a different set of pages in the restore

Yes.

> operation.  This would be an issue if pages had different characteristics such
> as being on different nodes.  Is that a real concern?

A good point. Now I see your concern, it is better to keep the same node
as before when error occurs.

> 
> I suppose we could change vmemmap_remap_pte to add pages to the head of
> the list?  I do not recall the reasoning behind adding to tail.

I think we could do this, the code will be a little simple. Actually, there
is no reason behind adding to tail (BTW, the first commit is introduced by
me, no secret here :-)).

Thanks.

> -- 
> Mike Kravetz